Lithographic printing is a printing system in which printing is conducted using a press plate with ink-receptive image areas and hydrophilic ink-repellent non-image areas. Namely, ink is applied to the sensitized areas, a dampening solution is applied to the hydrophilic areas and printing is made by utilizing mutual repulsion between ink and the dampening solution. It is important to feed the ink and the dampening solution to be applied to the surface of the press plate with proper ink-dampening solution balance. When the amount of the dampening solution applied is too large, ink is intensively emulsified and failure in transfer occurs, whereas when the amount of the dampening is too small, ink adheres to the non-image areas and scumming occurs.
For better control of the balance between ink and the dampening solution, dampening solutions usually contain isopropyl alcohol (IPA) to reduce surface tension, and further, various hydrophilic materials such as gum arabic, carboxymethyl cellulose (CMC), citric acid and various surfactants; an acid such as phosphoric acid as an affinitizing agent to remove oxides on the surface of the plate; and ammonium bichromate or nitrates as a corrosion inhibitor for the plate. IPA has been widely used because it has advantages in that, since IPA can reduce the surface tension of the dampening solution, the wetting of the hydrophilic non-image areas becomes good and the viscosity of the dampening solution is increased to thereby allow the smooth feed of the dampening solution to the surface of the plate to be made.
However, IPA comes under alcohols of the class 4 hazardous material specified in the Low of Japan because it is highly flammable substance, and, thus it must be handled with the greatest possible care against fire. Further, IPA is the class 2 organic solvent specified in "Yuki-Yozai Chudoku Yobo Kisoku" (the Rules for Prevention of Organic Solvent Poisoning) of Japan (hereinafter referred to as the "Rule for Organic Solvent") because it is highly harmful substance for the human body. Furthermore, the dampening solutions usually contain about 5 to 20% by weight of IPA and hence it is necessary to provide an apparatus for purifying working atmosphere.
To this end, JP-B-55-19757 (the term "JP-B" as used herein means an "examined Japanese patent publication") proposes dampening solutions for lithographic printing which contain propylene oxide or ethylene oxide alkyl ether surfactant in place of IPA, and JP-A-63-25093 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes dampening solutions for lithographic printing which contain polyethylene oxide surfactants in place of IPA.
Since the above-described surfactants are relatively safe for the human body so that the Rule for Organic Solvent is not applied to them. Furthermore, the surfactant is normally used in a content of 0.1 to 0.5% by weight in the dampening solution and the surfactant can certainly reduce the surface tension of the dampening solution with such a content. However, the transfer of the dampening solutions from a pan is poor in comparison with those containing IPA and the wetting of the hydrophilic non-image areas of the plate is not satisfactory.
Further, JP-A-3-63188 proposes dampening solutions containing ethylene oxide and/or propylene oxide adducts of 2-ethyl-1,3-hexanediol or ethylene oxide and/or propylene oxide adducts of acetylene alcohol or acetylene glycol (i.e., ether glycols which are alcohol derivatives). However, 2-ethyl-1,3-hexanediol itself, in particular, has poor solubility in water, and further the lipophilic (hydrophobic) propylene oxide adducts are scarcely soluble in water. Accordingly, the surface tension can not be lowered.
The surface tension can certainly be reduced by adding 0.5 to 50% by weight, based on the total weight of the solution, of the ethylene oxide or propylene oxide adducts of these compounds. However, the transfer of the dampening solutions from a pan is poor in comparison with those containing IPA and the wetting of the hydrophilic non-image areas of the plate is not satisfactory and, as a result, scumming on prints occurs.
Generally, ethylene glycol monobutyl ether is used as the organic solvent which is a substitute for IPA, and additives for lithographic dampening solutions which contain the same are on the market. Ethylene glycol monobutyl ether itself is a harmful substance for the human body so that it comes under the class 2 organic solvent specified in the Rule for Organic Solvent, but the dampening solution containing the same is not applied with the Rule for Organic Solvent because the dampening solutions usually contain it at a concentration of only 0.1 to 3% by weight. However, when ethylene glycol monobutyl ether is to be handled at a concentration of higher than 5% by weight in the preparation of the additives for the dampening solutions or in the adjustment of the dampening solutions, it is necessary to take an appropriate measure for preventing the worker from the hazard of the compound under the Rule for Organic Solvent. When ethylene glycol monomethyl ether is used as a glycol ether, the same measure must be taken. Accordingly, the manufacturers of the additives for the dampening solutions and the users thereof must take an appropriate measure for purifying working atmosphere.